A ConocoPhillips subsidiary, an oil spill by which polluted 5,500 square kilometres of water in China's northern Bohai Bay, said it had already sealed off the leaks ahead of an Aug. 31 deadline.

ConocoPhillips China said it had submitted a report to China's marine authority showing it has met requirements imposed at the end of last month to completely seal off oil spill sources and eliminate the risk of further leaks.

The oil leak at the Penglai 19-3 oilfield, China's biggest offshore oil field, which started in June is the most serious marine ecological incident in China, according to the State Oceanic Administration (SOA).

The report submitted to the SOA includes details of the methods used by ConocoPhillips to seal the leaks and includes third-party confirmation, the company said on its website (www.conocophillips.com.cn).

This report also includes the results of the company's internal investigation of the causes of the accident.

The company estimated that 115 cubic meters (700 barrels) of crude oil and 400 cubic meters (2,500 barrels) of mineral oil-based drilling mud have been leaked, it added.

Although there is no evidence that the oil slick itself made it to shore or soiled any local beaches, ConocoPhillips China crews continue to monitor the situation, collecting samples for testing, it added.

The North China Sea branch of SOA has finished its investigation into the ecological damage caused by the oil spill and is compiling an evaluation report, providing evidence for victims of the oil spill and for the government seeking compensation, state television said.

Fishermen in Hebei province were preparing to sue the U.S. firm for allegedly wiping out large numbers of scallops in the ocean, Chinese media have reported.

SOA said last week that ConocoPhillips will have to pay for the spill, adding that it would be ready to file suit against ConocoPhillips once it appoints a team of lawyers. It also said that the company would face strict measures if it failed to seal off the leaks on time.

ConocoPhillips owns a 49 percent stake in the oilfield and acts as the operator, while China's top offshore oil and gas producer CNOOC Ltd has a 51 percent stake.

Penglai 19-3 oilfield produces a total of 8.4 million tonnes a year (168,000 bpd), about 20 percent of total crude oil production in Bohai Bay.